19F Magic-angle Spinning NMR Research Articles

19F fast MAS (60–111 kHz) dipolar and scalar based correlation spectroscopy of organic molecules and pharmaceutical formulations

19F magic angle spinning (MAS) NMR spectroscopy is a powerful tool for characterization of fluorinated solids. The recent development of 19F MAS NMR probes, operating at spinning frequencies of 60–111 kHz, enabled analysis of systems spanning from organic molecules to pharmaceutical formulations to biological assemblies, with unprecedented resolution. Herein, we systematically evaluate the benefits of high MAS frequencies (60–111 kHz) for 1D and 2D 19F-detected experiments in two pharmaceuticals, the antimalarial drug mefloquine and a formulation of the cholesterol-lowering drug atorvastatin calcium. We demonstrate that 1H decoupling is essential and that scalar-based, heteronuclear single quantum coherence (HSQC) and heteronuclear multiple quantum coherence (HMQC) correlation experiments become feasible and efficient at the MAS frequency of 100 kHz. This study opens doors for the applications of high frequency 19F MAS NMR to a wide range of problems in chemistry and biology.

Composites of (C4F)n and (CF)n Synthesized by Uncatalyzed Fluorination of Graphite

A new solid-state 19F magic-angle spinning NMR signal at an isotropic 19F chemical shift of −53 ppm is measured from graphite fluoride synthesized by reaction of graphite with F2 at temperatures above 750 K with no catalyst. Two-dimensional NMR suggests the −53 ppm 19F NMR signal originates from covalent fluoromethanetriyl groups belonging to ordered (CyF)n bulk domains composited with the major (CF)n domains. Quantitative 19F and 13C NMR find y=4.32±0.64. DFT calculations of NMR chemical shifts for unsaturated fluorographene models show that a (C4F)n phase with fluorine bound covalently to a single side of the carbon layer best explains the observed NMR chemical shifts. We assign the new phase to this (C4F)n structure, which constitutes up to 15% of the carbon in our graphite fluoride composites. The (C4F)n content of the composite affects bulk electrochemical properties in a manner similar to graphite fluorides produced by conventional, catalyzed fluorination processes.

19F Dynamic Nuclear Polarization at Fast Magic Angle Spinning for NMR of HIV-1 Capsid Protein Assemblies.

We report remarkably high, up to 100-fold, signal enhancements in 19F dynamic nuclear polarization (DNP) magic angle spinning (MAS) spectra at 14.1 T on HIV-1 capsid protein (CA) assemblies. These enhancements correspond to absolute sensitivity ratios of 12-29 and are of similar magnitude to those seen for 1H signals in the same samples. At MAS frequencies above 20 kHz, it was possible to record 2D 19F-13C HETCOR spectra, which contain long-range intra- and intermolecular correlations. Such correlations provide unique distance restraints, inaccessible in conventional experiments without DNP, for protein structure determination. Furthermore, systematic quantification of the DNP enhancements as a function of biradical concentration, MAS frequency, temperature, and microwave power is reported. Our work establishes the power of DNP-enhanced 19F MAS NMR spectroscopy for structural characterization of HIV-1 CA assemblies, and this approach is anticipated to be applicable to a wide range of large biomolecular systems.

Insight into the Crystalline Structure of ThF4 with the Combined Use of Neutron Diffraction, 19F Magic-Angle Spinning-NMR, and Density Functional Theory Calculations.

Because of its sensitivity to the atomic scale environment, solid-state NMR offers new perspectives in terms of structural characterization, especially when applied jointly with first-principles calculations. Particularly, challenging is the study of actinide-based materials because of the electronic complexity of the actinide cations and to the hazards due to their radioactivity. Consequently, very few studies have been published in this subfield. In the present paper, we report a joint experimental-theoretical analysis of thorium tetrafluoride, ThF4, containing a closed-shell actinide (5f0) cation. Its crystalline structure has been revisited in the present work using powder neutron diffraction experiments. The 19F NMR parameters of the seven F crystallographic sites have been modeled using an empirical superposition model, periodic first-principles calculations, and a cluster-based all-electron approach. On the basis of the atomic position optimized structure, a complete and unambiguous assignment of the 19F NMR resonances to the F sites has been obtained.

Investigations of Uranyl Fluoride Sesquihydrate (UO2F2·1.57H2O): Combining 19F Solid-State MAS NMR Spectroscopy and GIPAW Chemical Shift Calculations.

High-resolution 19F magic-angle spinning (MAS) NMR spectra were obtained for the uranium-bearing solid uranyl fluoride sesquihydrate (UO2F2·1.57H2O). While there are seven distinct crystallographic fluorine sites, the 19F NMR spectrum reveals six peaks at -33.3, 9.1, 25.7, 33.0, 39.0, and 48.2 ppm, with the peak at 33.0 ppm twice the intensity of all the others and therefore corresponding to two sites. To assign the peaks in the experimental spectra to crystallographic sites, 19F chemical shifts were calculated using the gauge including projector augmented waves (GIPAW) plane-wave pseudopotential approach for a DFT-optimized crystal structure. The peak assignments from DFT are consistent with two-dimensional double-quantum 19F MAS NMR experiments.

Comprehensive Solid-State Characterization of Rare Earth Fluoride Nanoparticles

The combination of multinuclear solid-state NMR spectroscopy and powder X-ray diffraction has been applied to characterize the octahedron-shaped crystalline nanoparticle products resulting from an inverse micelle synthesis. Rietveld refinements of the powder X-ray diffraction data from the nanoparticles revealed their general formula to be (H3O)Y3F10·xH2O. 1H magic-angle spinning (MAS) NMR experiments provided information on sample purity and served as an excellent probe of the zeolithic incorporation of atmospheric water. 19F MAS NMR experiments on a series of monodisperse nanoparticle samples of various sizes yielded spectra featuring three unique 19F resonances arising from three different fluorine sites within the (H3O)Y3F10·xH2O crystal structure. Partial removal of zeolithic water from the internal cavities and tunnels of the nanoparticles led to changes in the integrated peak intensities in the 19F MAS NMR spectra; the origin of this behavior is discussed in terms of 19F longitudinal relaxation. 19F–89Y variable-amplitude cross-polarization (VACP) NMR experiments on both stationary samples and samples under MAS conditions indicated that two distinct yttrium environments are present, and on the basis of the relative peak intensities, the population of one of the two sites is closely linked to the nanoparticle size. Both 19F MAS and 19F–89Y VACP/MAS experiments indicated small amounts of an impurity present in certain nanoparticles; these are postulated to be spherical amorphous YF3 nanoparticles. We discuss the importance of probing molecular-level structure in addition to microscopic structure and how the combination of these characterization methods is crucial for understanding nanoparticle design, synthesis, and application.

Selective Formation of Hydrogen and Hydroxyl Radicals by Electron Beam Irradiation and Their Reactivity with Perfluorosulfonated Acid Ionomer

Selective formation and reactivity of hydrogen (H(•)) and hydroxyl (HO(•)) radicals with perfluorinated sulfonated ionomer membrane, Nafion 211, is described. Selective formation of radicals was achieved by electron beam irradiation of aqueous solutions of H2O2 or H2SO4 to form HO(•) and H(•), respectively, and confirmed by ESR spectroscopy using a spin trap. The structure of Nafion 211 after reaction with H(•) or HO(•) was determined using calibrated (19)F magic angle spinning NMR spectroscopy. Soluble residues of degradation were analyzed by liquid and solid-state NMR. NMR and ATR-FTIR spectroscopy, together with determination of ion exchange capacity, water uptake, proton conductivity, and fluoride ion release, strongly indicate that attack by H(•) occurs at the tertiary carbon C-F bond on both the main and side chain; whereas attack by HO(•) occurs solely on the side chain, specifically, the α-O-C bond.

Solid-state nuclear magnetic resonance studies of electrochemically discharged CF x

Electrochemical studies of three types of CF x (F – fiber based, C – petroleum coke based, G – graphite based) have demonstrated different electrochemical performances in previous work, with fiber based CF x delivering superior performance over those based on petroleum coke and graphite. 13C and 19F MAS (magic angle spinning) NMR techniques are employed to identify the atomic/molecular structural factors that might account for differences in electrochemical performance among the different types of CF x . Small quantitative variations of covalent CF and LiF are noted as a function of discharge and sp 3 bonded carbons are detected in discharged F type of CF x .

Conformations of Silica-Bound (Pentafluorophenyl)propyl Groups Determined by Solid-State NMR Spectroscopy and Theoretical Calculations

The conformations of (pentafluorophenyl)propyl groups (-CH(2)-CH(2)-CH(2)-C(6)F(5), abbreviated as PFP), covalently bound to the surface of mesoporous silica nanoparticles (MSNs), were determined by solid-state NMR spectroscopy and further refined by theoretical modeling. Two types of PFP groups were described, including molecules in the prone position with the perfluorinated aromatic rings located above the siloxane bridges (PFP-p) and the PFP groups denoted as upright (PFP-u), whose aromatic rings do not interact with the silica surface. Two-dimensional (2D) (13)C-(1)H, (13)C-(19)F and (19)F-(29)Si heteronuclear correlation (HETCOR) spectra were obtained with high sensitivity on natural abundance samples using fast magic angle spinning (MAS), indirect detection of low-gamma nuclei and signal enhancement by Carr-Purcell-Meiboom-Gill (CPMG) spin-echo sequence. 2D double-quantum (DQ) (19)F MAS NMR spectra and spin-echo measurements provided additional information about the structure and mobility of the pentafluorophenyl rings. Optimization of the PFP geometry, as well as calculations of the interaction energies and (19)F chemical shifts, proved very useful in refining the structural features of PFP-p and PFP-u functional groups on the silica surface. The prospects of using the PFP-functionalized surface to modify its properties (e.g., the interaction with solvents, especially water) and design new types of the heterogeneous catalytic system are discussed.

Impact of Reduction on the Properties of Metal Bisdithiolenes: Multinuclear Solid-State NMR and Structural Studies on Pt(tfd)2 and Its Reduced Forms

Transition-metal dithiolene complexes have interesting structures and fascinating redox properties, making them promising candidates for a number of applications, including superconductors, photonic devices, chemical sensors, and catalysts. However, not enough is known about the molecular electronic origins of these properties. Multinuclear solid-state NMR spectroscopy and first-principles calculations are used to examine the molecular and electronic structures of the redox series [Pt(tfd)(2)](z-) (tfd = S(2)C(2)(CF(3))(2); z = 0, 1, 2; the anionic species have [NEt(4)](+) countercations). Single-crystal X-ray structures for the neutral (z = 0) and the fully reduced forms (z = 2) were obtained. The two species have very similar structures but differ slightly in their intraligand bond lengths. (19)F-(195)Pt CP/CPMG and (195)Pt magic-angle spinning (MAS) NMR experiments are used to probe the diamagnetic (z = 0, 2) species, revealing large platinum chemical shielding anisotropies (CSA) with distinct CS tensor properties, despite the very similar structural features of these species. Density functional theory (DFT) calculations are used to rationalize the large platinum CSAs and CS tensor orientations of the diamagnetic species using molecular orbital (MO) analysis, and are used to explain their distinct molecular electronic structures in the context of the NMR data. The paramagnetic species (z = 1) is examined using both EPR spectroscopy and (13)C and (19)F MAS NMR spectroscopy. Platinum g-tensor components were determined by using solid-state EPR experiments. The unpaired electron spin densities at (13)C and (19)F nuclei were measured by employing variable-temperature (13)C and (19)F NMR experiments. DFT and ab initio calculations are able to qualitatively reproduce the experimentally measured g-tensor components and spin densities. The combination of experimental and theoretical data confirm localization of unpaired electron density in the pi-system of the dithiolene rings.

NMR evidence of LiF coating rather than fluorine substitution in Li(Ni 0.425Mn 0.425Co 0.15)O 2

A series of “Li 1+ z /2(Ni 0.425Mn 0.425Co 0.15) 1− z /2O 2− z F z ” materials was prepared by a coprecipitation route and their structure was characterized using X-ray diffraction (XRD), as well as 7Li and 19F Magic Angle Spinning (MAS) NMR spectroscopy. Two hypotheses were considered: (i) formation of layered oxyfluoride materials and (ii) formation of a mixture between the layered material and LiF. Structural parameters were refined by the Rietveld method, using XRD diffraction data. The refinement results did not allow us to choose between these two hypotheses: no significant change in crystallinity and structural parameters was observed irrespective of the fluorine ratio. 7Li and 19F MAS NMR analyses showed signals with isotropic positions characteristic of LiF, but envelopes characteristic of very strong dipolar interactions with the electron spins of the material, demonstrating that LiF was not incorporated into the layered oxide structure but was instead present as a coating.

Structure and dynamics of a vinylidene fluoride oligomer and its cyclodextrin inclusion compounds as studied by solid-state 19F MAS and 1H → 19F CP/MAS NMR spectroscopy

The molecular structure and dynamics of a vinylidene fluoride oligomer telomerized by carbon tetrachloride (Cl-OVDF) and its inclusion compound (IC) with β-cyclodextrin (β-CD) have been investigated using solid-state 19F magic angle spinning (MAS) and 1H→19F cross-polarization (CP)/MAS NMR spectroscopy. The preferential IC formation of the lower-molecular-weight components with β-CD was used to refine as-received Cl-OVDF. The refined Cl-OVDF with larger molecular weight readily takes γ-form (tttg+tttg−) conformation, and it also forms ICs with β-CD (Cl-OVDF/β-CD IC) under a certain condition. 19F MAS NMR indicates that Cl-OVDF chains virtually isolated in the β-CD cavities take no specific conformations even at −40°C. The temperature dependence of the magnetic relaxation times (T1F, T1ρF) indicates that the Cl-OVDF chains in ICs undergo molecular motions similar to the amorphous phase in the bulk, although the intramolecular spin diffusion among 19F nuclei is more significant in the former because of the one-dimensional confinement.

A Solid-State NMR, X-ray Diffraction, and Ab Initio Investigation into the Structures of Novel Tantalum Oxyfluoride Clusters

A series of tantalum oxyfluoride materials containing the [Ta4F16O4]4− and [Ta8F24O12]8− anion clusters have been synthesized and characterized using X-ray diffraction (XRD) and solid-state nuclear magnetic resonance (SSNMR) spectroscopy. The structure of both tantalum oxyfluoride materials display octahedrally bonded tantalum atoms with bridging oxygen and terminal fluoride atoms. The [Ta4F16O4]4− cluster is an eight-membered ring, whereas the [Ta8F24O12]8- cluster forms a cagelike structure. Solid-state dynamics of these clusters were explored by monitoring the impact of temperature on the one-dimensional (1D) 19F magic angle spinning (MAS) NMR, 13C cross-polarization (CP) MAS NMR, and two-dimensional (2D) double quantum (DQ) 19F MAS NMR spectra. The DQ 19F NMR correlation experiments allowed the through space connectivity between the different resolved fluorine environments to be determined, thus aiding in the spectral assignment and structural refinement of these materials. Ab initio 19F NMR chemical shift calculations were used to assist in the interpretation of the 19F NMR spectra. The influence of scalar relativistic and Ta−F spin–orbit coupling on the 19F NMR shielding calculation arising from bonding to tantalum atoms is also addressed.

Anion diffusivity in highly conductive nanocrystalline BaF2:CaF2 composites prepared by high-energy ball milling

We report on the fluorine diffusivity in nanocrystalline BaF2 and CaF2 as well as in BaF2:CaF2 composites prepared by high-energy ball milling. Mechanical treatment of polycrystalline BaF2 together with CaF2 results in a nanocrystalline composite with an unexpectedly high dc conductivity of about 0.1 mS cm−1 at 450 K. X-ray diffraction and 19F magic angle spinning NMR measurements at extraordinary high spinning speeds reveal that this can be explained inter alia by the interplay of structural disorder and the formation of a mixed (Ba,Ca)F2 phase.

Direct Solid-State NMR Spectroscopic Evidence for the NH4AlF4 Crystalline Phase Derived from Zeolite HY Dealuminated with Ammonium Hexafluorosilicate

Multinuclear (1)H, (19)F, and (27)Al MAS (magic angle spinning) and corresponding 2D HETCOR (heteronuclear correlation) NMR spectroscopy, in combination with powder XRD measurements, provide the direct evidence for the NH(4)AlF(4) crystalline phase, which was formed from zeolite HY dealuminated with an aqueous (NH(4))(2)SiF(6) solution at 80 degrees C. The NH(4)AlF(4) crystalline phase exhibits a characteristic second-order quadrupolar-induced (27)Al NMR line shape spreading from 0 to -90 ppm (in a magnetic field of 11.7 T) and two (19)F resonances at -151 and -166 ppm in the (19)F NMR spectrum. An( 27)Al quadrupolar coupling constant (C(Q)) of 9.5 MHz and an asymmetry parameter (eta) of 0.1 were identified, for the first time, for the NH(4)AlF(4) crystalline phase observed. On the basis of the (19)F{(27)Al} TRAPDOR (transfer population in double resonance) NMR results, the (19)F resonances at -151 and -166 ppm are therefore assigned to (19)F spins associated with the fluorines in the terminal Al-F and the bridging Al-F-Al groups, respectively.

Dynamic Effects in MAS and MQMAS NMR Spectra of Half-Integer Quadrupolar Nuclei: Calculations and an Application to the Double Perovskite Cryolite

Dynamic processes such as chemical exchange or rotations between inequivalent orientations can affect the magic-angle spinning (MAS) and the multiple-quantum (MQ) MAS NMR spectra of half-integer quadrupolar nuclei. The present paper discusses such dynamic multisite MAS and MQMAS effects and applies them to study the dynamic processes that occur in the double perovskite cryolite, Na3AlF6. Dynamic line shape simulations invoking a second-order broadening of the central transition and relying on the semiclassical Bloch-McConnell formalism for chemical exchange were performed for a variety of exchange models possessing different symmetries. Fitting experimental variable-temperature cryolite 23Na NMR data with this formalism revealed that the two inequivalent sodium sites in this mineral undergo an exchange characterized by a broad distribution of rates. To further assess this dynamic process a variety of 27Al and 19F MAS NMR studies were also undertaken; quantitative 27Al-19F dipolar coupling measurements then revealed a dynamic motion of the AlF6 octahedra that were qualitatively consistent with predictions stemming from molecular dynamic simulations on this double perovskite.

Post-synthesis alumination of mesoporous silica SBA-15 with high framework aluminum content using ammonium hexafluoroaluminate

Incorporation of high aluminum contents into the silica framework of SBA-15 was achieved by treating SBA-15 with an aqueous (NH 4) 3AlF 6 solution at room temperature under alkaline conditions. Characterization by powder X-ray diffraction (XRD), N 2 adsorption–desorption, transmission electron microscopy (TEM), FTIR, and multinuclear solid-state NMR measurements was carried out to evaluate the efficiency of this alumination method. 27Al magic angle spinning (MAS), 1H/ 27Al TRAnsfer of Population in DOuble Resonance (TRAPDOR) NMR, and cumene cracking measurements demonstrated that the aluminated SBA-15 materials contained exclusively tetrahedrally coordinated aluminum and exhibited well-developed Brønsted acidity. A high level of framework aluminum substitution, up to an Si/Al ratio near 5, without any significant loss in the textural properties of SBA-15, was achieved via the present post-synthesis route. 19F MAS NMR was helpful in gaining more insights into the mechanism of the present alumination process.

Investigating the Crystalline Structure of Poly(vinylidene fluoride) (PVDF) in PVDF/Silica Binary and PVDF/Poly(methyl methacrylate)/Silica Ternary Hybrid Composites Using FTIR and Solid-State 19F MAS NMR Spectroscopy

We investigated the crystalline structures of poly(vinylidene fluoride) (PVDF) in PVDF/silica (SiO2) binary and PVDF/poly(methyl methacrylate) (PMMA)/SiO2 ternary hybrid composite films using infrared (IR) analysis and solid-state, high-speed magic-angle-spinning (MAS) 19F NMR spectroscopy. These hybrid films were prepared by sol−gel processes. We used three different blending sequences to prepare the ternary hybrid composite films: (1) The “Type 1” hybrid was prepared from a one-pot mixture of PVDF, PMMA, and tetraethoxysilane (TEOS), the silica precursor, in dimethylacetamide (DMAc). (2) The “Type 2” hybrid composite was prepared by first forming a PVDF/silica hybrid and then mixing it with PMMA. (3) The “Type 3” hybrid composite was prepared by mixing a preformed PMMA/silica hybrid with PVDF. The crystallinity of the PVDF/SiO2 hybrid composite films decreased upon increasing the silica content. By using the different methods to prepare the PVDF/PMMA/SiO2 ternary hybrid composite films, we were able to assign the γ-phase of PVDF crystals in the solid-state 19F MAS NMR spectrum, which shows seven resonances that are attributable to an amorphous domain (−88 ppm), crystalline domains (−101.3, −93.7, −84.2, and −79.6 ppm), and regioirregular structures (−112.4 and −110.4 ppm). 19F MAS NMR spectra of the Type 2 and Type 3 PVDF/PMMA/SiO2 hybrid composite films show two new resonances, assignable to the γ-phase at lower (−101.3 ppm) and higher (−84.2 ppm) frequencies, which show long 19F spin−lattice relaxation times in the rotating frame (T1ρF) and chemical shifts that are significantly different from those of α- and β-phases of the PVDF crystals. The disappearance of the α-phase of PVDF crystals in the Type 2 and Type 3 hybrid composite films can be explained by the large silica particles in the PVDF/PMMA matrix hindering crystallization of PVDF. Solid-state 19F spin-lock NMR spectroscopy experiments also indicate significant differences exist between the values of T1ρF of the crystalline and amorphous domains depending on the method of hybrid preparation.

Hydrothermal Synthesis, Single-Crystal Structure Analysis, and Solid-State NMR Characterization of Zn2(OH)0.14(3)F0.86(3)(PO4)

The zinc fluoro phosphate Zn2F(PO4) has been produced by hydrothermal synthesis employing hydrofluoric acid as a mineralizer in a H2O or D2O medium. A single-crystal X-ray synchrotron diffraction analysis of Zn2F(PO4) shows that the zinc fluoro phosphate is monoclinic, a=9.690(1), b=12.793(1), and c=11.972(1) Å, β=108.265(1)°, space group P21/c, No. 14, Z=16. Reflections hkl with k=2n+1 are weak but significant and the structure shows pseudosymmetry. Zn2F(PO4) has the wagnerite-type M2F(XO4) structure with four Zn atoms each coordinated to four O atoms and one F atom while four other Zn atoms are coordinated to four O atoms and two F atoms. A difference Fourier map, calculated from the single-crystal X-ray data, shows additional electron density close to the four fluorine atoms, indicating a possible partial substitution of F− by OH− ions. This is unambiguously confirmed by 31P–{1H} cross-polarization magic-angle spinning (MAS) and by 1H/2H MAS NMR spectroscopy. The narrow line width observed for the 1H resonance and the unique set of 2H quadrupole coupling parameters (obtained for the Zn2F(PO4) sample using D2O as medium) show that 1H/2H is present as OH(D) groups rather than as water of crystallization in the structure. Quantitative 1H MAS NMR analysis shows that the composition of the sample is Zn2(OH)0.14(3)F0.86(3)(PO4). The high-speed 19F MAS NMR spectrum exhibits two resolved resonances with equal intensity, which are ascribed to an overlap of resonances from the four distinct fluorine sites in Zn2(OH)0.14(3)F0.86(3)(PO4).

19F to 1H and 1H to 19F Cross polarization MAS NMR of trifluoroethanol chemisorbed on magnesium oxide

19 F magic angle spinning NMR studies of surfaces are greatly facilitated by 1H to 19F cross polarization and 1H decoupling.